1. Field of the Invention
The present invention generally relates to a cooling device, and in particular to a cooling device for cooling an object, which emits a large amount of heat such as a laser diode (LD) array, to constitute a light source device therewith.
2. Description of Related Art
A laser diode array is used as a light source for exciting a laser medium of a solid-state laser oscillator of high power. Laser oscillation of high efficiency is achieved by exciting the solid-state laser medium by the laser diode array which emits a light beam of acute spectrum instead of a conventional light source such a xenon lamp. A two-dimensional laser diode array (surface illuminating device) formed by stacking one-dimensional laser diode arrays (laser diode bars) having a length of approximately 1 cm and an output power of several tens watt is typically used as the exciting light source of the high power solid-state laser.
For obtaining a high-power light from the LD bars, a cooling device for cooling the LD bars is necessary since a large amount of heat not less than output power of the emitted light is generated by the LD bars. It is required to stack a large number of LD bars in order to obtain high-power output and also it is required to make a pitch of stacking small in order to increase a density of the output power.
Thus, the cooling device for cooling the LD bar is required to allow stacking a larger number of stacked LD bars with a small pitch to form a surface illuminating device with high power and high density of output power. As a flow rate of coolant necessary for cooling the larger number of LD bars increases, there arises a problem of increasing of a pressure loss of the coolant in the cooling device to fail in securing the required flow rate of coolant, to lower a cooling performance of the cooling device.
In view of the above requirements and problems, it is desirable to reduce the pressure loss in the cooling device without increasing a thickness of the device and without lowering a mechanical strength of the device constituted by thin layers. Also, it is necessary to reduce a manufacturing cost of the device since a large number of LD bars are used in the device.
FIG. 1 is an exploded view of a conventional surface light emitting device 1 having a plurality of LD bars. In FIG. 1, a plurality of LD bars 2 are thermally connected to respective cooling devices 3 and the coolant is introduced from a common inlet opening 4 into respective flow passages formed in the cooling devices 3 to flow immediately below the respective LD bars 2 and discharged from an outlet opening 5. With this arrangement, thermal resistance between the LD bars and the coolant is reduced to suppress a temperature rise of the LD bars 2.
In FIG. 1, the LD bras 2 are depicted with exaggerated gaps therebetween, and sealing members for preventing leakage of the coolant from the cooling devices and leads for electrically connecting electrodes of the LD bars 2 which are not connected with the cooling device 3 are not shown.
FIG. 2 is an exploded view of one cooling device 3 for cooling one LD bar 2 in the plurality of cooling devices 3 as shown in FIG. 1. This arrangement is described in German Patent Publication DE 4315580 A1.
Referring to FIG. 2, the cooling device 3 comprises five lamellar plates of first to fifth lamellar plates 6-10 with respective inlet openings 4 and outlet openings 5 formed at corresponding positions. The second lamellar plate 7 is arranged on the first lamellar plate 6 and has a coolant path 12 communicating with the inlet opening and extending from the inlet opening 4 to a front side 11 of the cooling device 3 with its width expanding. The third lamellar plate 8 arranged on the second lamellar plate 7 has a slit 13 formed separately from the inlet opening 4 and the outlet opening 5 along the front side 11 of the cooling device 3 and functions as a coolant path. The fourth lamellar plate 9 arranged on the third lamellar plate 8 has micro channels 14 formed along the front side 11 to be corresponding to the slit 13 and a coolant path 12 communicating with the outlet opening 5 and extending from the micro channels 14 to the outlet opening 5 with its width expanding. The fifth lamellar plate 10 is arranged on the fourth lamellar plate 10.
Each of the thin plates 6-10 is made of material having high thermal conductivity such as copper and the laminated plates 6-10 forms a flow passage of the coolant introduced from the inlet opening 4 to the outlet opening 5 through the micro channels 14. The LD bar, which is arranged on the uppermost layer plate 10 along a front face 11 of the cooling device 3, is cooled by the cooling device 3. The micro channels 14 are formed by laser machining etc. to have approximately 20 μm so as to suppress reduction of a heat exchange efficiency by a boundary layer of the coolant.
In this arrangement shown in FIG. 2, since the thin layer plate 6-10 are formed by the laser machining and stamping, a relatively large number, i.e., five or more of thin plates are necessary for forming one cooling device 3 to raise the number of parts necessary for the cooling device, and a width of the flow passage other than a part at the micro channels is large.
Therefore, thickness of the thin layer plate can not be made thinner in view of mechanical strength of the cooling device and the number of thin plates is large to make the cooling device thick. Further, since it is difficult to form the micro channel by the stamping or chemical etching, the micro channel has to be formed by the laser machining. As the laser machining of high cost is necessary, a cost of the manufacturing of the cooling device is made high.
FIG. 3 is an exploded view of another cooling device for cooling one LD bar, as disclosed in U.S. Pat. No. 5,105,429. As shown in FIG. 3, a cooling device 15 comprises (1) a lower thin layer plate 17 having an inlet opening 4, an outlet opening 5 and a coolant flow path 16 extending from the inlet opening 4 with its width expanding to a front face 11 of the cooling device 15; (2) a middle thin layer plate 18 disposed on the lower thin layer plate 17 and having an inlet opening 4 and an outlet opening 5 corresponding to the inlet opening 4 and the outlet opening 5 of the lower thin layer plate 17 and a slit 13 communicating with the front portion of the flow path 16 of the lower thin layer board 17 to function as a flow path separately from the inlet path 4 and the outlet path; (3) a micro channel formed along the front face 11 of the cooling device; (4) an upper thin layer plate 19 having a coolant flow path 16 extending from the micro channel to the outlet opening 5 with its width expanding to communicate with the micro channel with an outlet opening 5.
The lower thin plate 17 and the upper thin plate 18 are formed by material of silicon as described in U.S. Pat. No. 5,105,429. These boards are joined together to be laminated to form the flow passage for guiding the coolant from the inlet opening 4 to the outlet opening 5 through the micro channel.
The LD bar 2 provided on the upper thin layer plate 19 along an obverse surface 11 of the cooling device 3 is cooled by the cooling device 3. The micro channel 14 is formed by anisotropic etching of silicone to have a width of approximately 25 μm and a depth of 125 μm so as to suppress reduction of a heat exchange efficiency by a boundary layer of the coolant.
It is necessary to form the thin layer plates from mono-crystal as material for forming the micro channel by the anisotropic etching, and therefore a high pressure can not be applied in the direction of lamination of the thin layer boards since a mechanical strength of the cooling device is low, to make a possibility of leakage of coolant.
Further, since cost of the anisotropic etching is high and the number of parts are large, to raise a manufacturing cost of the cooling device.
FIG. 4 shows another cooling device as disclosed in Japanese Patent Publication No. 10-209531. FIG. 4 is an exploded view of a cooling device 20 for cooling one LD bar. The cooling device 20 comprises three plate members 21-23 having high heat conductance. An upper plate member 23 has a coolant flow path 24 having the same pattern as that of a flow path formed in a lower plate member 21.
The coolant flow path is formed by a plurality of channels 26 separated by ridges 25. The ridges are joined with the middle plate member 22 mechanically and thermally and a plurality of small through holes are formed in the middle plate member 22 instead of a slit so that bridges between the through holes contributes in heat conductivity and in preventing deformation of the middle plate member 22.
With the above arrangement, thermal connection between the three plate members 21-23 is improved so that heat generated by the LD bar is effectively diffused over a wide region of the cooling device 20, to realize an excellent cooling performance without forming micro channels which incurs high cost. Further, the cooling device 20 constituted by only three plate members has a high mechanical strength. However, there remain problems of the pressure loss of the coolant caused by relatively narrow portions, discontinuity of direction of the flow passage and confluent and diffluent of the coolant flow.